thinking about installing a exhaust bypass system

[The Wizard Of Z]

well a friend of mine showed me this iron cast cut out, its basically a y shape pipe that you install before the cat and comes with a switch that basically lets you run from normal exhaust for city streets and highways and then when you want you can switch it to run threw the pipes second open witch is just running open headers basically, i was just wondering if this would be safe or at least not harmful for my car, ive ran open headers before and it sounded really mean , i mean i think it should be ok without damaging it, i dunno if the lack of back power is gonna be an issue or not, i mean i running a newly built 280 z engine thats been bored out to a 300 liter and has the dual round top su's and im putting out 300hp, so if anyone knows wether or not it would be alright to add this thing in, i would appreciate any info, thanks